• Line Winther
In this master project the geomorphological, water chemical and biological effects causes by the removal of a dam in a considerably large Danish stream (Gudenåen) is being studied. The place for these studies is Vilholt Mølle (Vilholt Mill), which was the last total blockage of Gudenåen and at the same time the last large blockage south of the city Silkeborg. The overall purpose of this restoration project was to recreate the natural flow of the stream running past the mill, allowing water dwelling animals (Macroinvertebrates) and fish to pass. Furthermore it was expected that the removal of the dam would have a positive physical influence on the stowed stretch of water upstream from the mill, causing the structure of this stretch to resemble the stretch downstream. Particularly Vilholt Stryget, approximately 1 km. downstream from the mill, is of great importance to the stream as this area consists of several important spawning areas making it crucial that this area be affected as little as possible by the removal of the dam. By studying the development in geomorphology and available habitats, taking samples to register the macroinvertebrates, while at the same time measuring oxygen and temperature of the stream, it was possible to observe the repercussions of the restoration. A surveillance program was followed which included measurements of: cross sections, stream velocities, the bottom levels and structures, as well as the composition of macroinvertebrates at 3 points upstream of the mill and 3 points downstream, one of these points was a part of Vilholt Stryget. Measurements and counts were made before the removal of the dam and were repeated at one month intervals for a period of approximately 5 months. The oxygen levels and temperatures were also measured consistently downstream prior to and following the removal of the dam, and water samples were studied to determine the contents of suspended matter and organic matter. In addition to the measurements a hydraulic modeling of the affected stretch of water was made, before and after the removal of the dam, so that the water depth of the inserted cross section could be determined by a given water flow. The relationship between water depth and water flow through the inserted cross section was transferred to CASiMiR-Fish, subsequently, a model for determining where at the new stretch adult trout and trout fry would prefer to stand were prepared. The measurements indicate that a significant alteration of the physical aspects upstream of Vilholt Mølle occurs. The average water level across a point of measurement decreases by up to 0.15 m, the slope of the stream increases from 0.26% to 0.75% and the velocity of the stream in the area studied doubles in some areas (from 0.6 m/s to 1.3 m/s). This increase in velocity leads to the removal of large amounts of sand from the bottom, which brings about a fall in the bottom level by up to 0.7 m. These physical alterations cause a change in the composition of the macroinvertebrates, the collected samples do, however, indicate that this is mostly caused by the macroinvertebrates being swept away by the stronger currents. The final samples, collected in March, indicate that the composition of macro invertebrates is being restored and that includes more pure water species than prior to the restoration. The measurements made downstream from the mill indicate that the grit chamber unfortunately has not had the desired and expected effect. At points 4 and 5 approximately 550 m. downstream from the dam at Vilholt Mølle large amounts of sand has been deposited and the bottom level has increased by up to 0.5m. This has affected the macroinvertebrate population slightly, although not as drastically as seen on the upstream stretch, since the sedimentation at points 4 and 5 was relatively fine grained to begin with and the water velocities has not changed as drastically as they have upstream. The population of macroinvertebrates at point 6 on Vilholt Stryget is estimated to have been much more affected by the restoration. Unfortunately no measurements/counts from before the removal of the dam are available for this point, since no influence was expected at this A-targetted stretch. The measurements and counts made following the removal of the dam have shown that the stretch is covered in sand and that the macroinvertebrate population is not as varied and does not consist of as many species as described by Jensen and Søholm (2003) among others. At the time of the last counts at point 6, the macroinvertebrate population had been so affected that the calculation of DVFI value results in the lowest score calculated during the entire period for the entire stretch of water, upstream and downstream from Vilholt Mølle. In the short term the removal of the dam has had many consequenses on the composition of macroinvertebrates upstream as well as downstream from the mill. Measurements suggest that especially on the otherwise diverse and varied stretch at Vilholt Stryget, the large amounts of deposited sand has had devastating effects on the macroinvertebrate population. How much this will affect the spawning of the Trout is not yet known, but the amounts of sand deposits observed at point 6 will most likely have a more or less negative influence. The long term effects of the removal of the dam will undoubtedly be positive in regards to the stretch at Vilholt Mølle. An increase in macroivertebrates in the upstream stretch was already observed during the last samplings in March. Furthermore the physical conditions on this stretch have become more varied, now consisting of both finely grained and large grained sediments as well as pools and riffles, the currents are also varied. Vilholt Stryget downstream from the mill is still very much affected by the removal of the dam at the last measurements in March, but a previous experience, in which Vilholt Stryget was covered in sand following a breakage in the dam, shows that Vilholt Stryget has the ability to restore both physical and biological aspects rather rapidly, which will hopefully be the case this time as well.
Publication date2009
Number of pages187
Publishing institutionAalborg Universitet
ID: 17696943